Aluminum-silicon alloy article and method for its production

ABSTRACT

A squeeze formed aluminum-silicon cylinder liner for an internal combustion engine is produced from a melt consisting essentially of the following composition by weight: silicon 14% to 16%, copper 1.9% to 2.2%, nickel 1.0% to 1.4%, magnesium of 0.4% to 0.55%, iron 0.6% to 1.0%, manganese 0.3% to 0.6%, silicon modifier 0.02% to 0.1%, with the balance being aluminum and any unavoidable impurities, the as-formed article having an essentially eutectic microstructure containing not more than 10% of primary alpha-aluminum dendrites and being substantially free from intermetallic particles exceeding 10μ in diameter. The growth rate R of the solid phase during solidification is from 1,000 to 2,500 μ/s and the temperature gradient G at the solid/liquid interface, expressed in °C./cm is such that the ratio G/R is from 100 to 1,000° Cs/cm 2 .

This invention relates to an aluminium-silicon alloy article produced bya squeeze forming process and particularly, but not essentially, relatesto a squeeze formed aluminium-silicon alloy cylinder liner for aninternal combustion engine.

It is known that cylinder liners can be produced by squeeze formingaluminium alloys but, in order to provide adequate wear properties, theinternal bores of such liners have hitherto required plating with anickel-silicon carbide composite coating. Generally, such a coatingprocess adds an unacceptable production cost to such liners.

It has been proposed in GB-A-2085920 of Comalco Limited to utilise acast aluminium-silicon alloy in the production of various automotivecomponents wherein the alloy has an essentially eutectic microstructurecontaining not more than 10% of primary alpha-aluminium dendrites andbeing substantially free from intermetallic particles exceeding 10μ indiameter. Further discussion regarding the alloy, the subject of thispatent specification, is to be found in SAE Technical Paper Series840123 and 860558 wherein the alloy, identified by Comalco Limited as3HA, is described as being castable by gravity, low pressure or highpressure casting methods.

It is to be understood from the above mentioned patent specification andpapers that the properties of 3HA alloy are determined by the correctmicrostructure which in turn is produced by the specific alloycomposition and its solidification parameters. For example, in thepatent specification it is stated that strengthening of the matrix isenhanced by the presence of stable manganese and zirconium containingparticles. It is also stated that titanium, because of its known grainrefining characteristics, is added to improve castability and to improvethe mechanical properties of the alloy.

We have discovered that articles of an aluminium-silicon alloy havingsimilar desirable characteristics to those of the 3HA alloy can beproduced from an alloy melt of a different composition to that of 3HAand with different solidification parameters. Hence it is an object ofthe present invention to provide a new or improved article of analuminium-silicon alloy and the method for its production.

In accordance with the invention there is provided a squeeze formedaluminium-silicon alloy article produced from a melt consistingessentially of the following composition by weight:

    ______________________________________                                        Silicon             14% to 16%                                                Copper              1.9% to 2.2%                                              Nickel              1.0% to 1.4%                                              Magnesium           0.4% to 0.55%                                             Iron                0.6% to 1.0%                                              Manganese           0.3% to 0.6%                                              Silicon Modifier    0.02% to 0.1%                                             ______________________________________                                    

with the balance being aluminium and any unavoidable impurities, theas-formed article having an essentially eutectic microstructurecontaining not more than 10% of primary alpha-aluminium dendrites andbeing substantially free from intermetallic particles exceeding 10μ indiameter.

The weight of silicon modifier in the melt may be within the range 0.02%to 0.08% and, preferably, the silicon modifier is strontium.

Also in accordance with the invention there is provided a method ofproducing the squeeze formed article as described above wherein, duringthe squeeze forming operation, the melt is solidified under conditionsof sustained temperature and pressure such that the growth rate R of thesolid phase during solidification is from 1,000 to 2,500 μ/s and thetemperature gradient G at the solid/liquid interface, expressed in°C./cm is such that the ratio G/R is from 100 to 1,000 °Cs/cm².

The method may comprise the additional step of subjecting the as-formedarticle to a full heat treatment process being a solution treatmentincluding heating and holding the article at a first temperature, waterquenching the article and reheating and holding the article at a secondtemperature which is lower than said first temperature.

The squeeze formed aluminium-silicon alloy article referred to aboveconveniently comprises a cylinder liner for an internal combustionengine although squeeze formed articles produced in accordance with theinvention may comprise other components where the wear resistantproperties are of importance; thus, for example but without limitation,the article may comprise a reciprocable piston for an internalcombustion engine or a compressor, or a brake disc or drum or brakecylinder for automotive application.

As will be known to those skilled in the art, the technique of squeezeforming essentially comprises introducing liquid metal into a first partof a mould, closing the mould under pressure so that the liquid metalfills the mould cavity without entrapping air, maintaining the metalunder pressure whilst solidification takes place so as to ensure thatany shrinkage cavities which may form are closed and filled, and thenopening the mould and removing the formed article.

The squeeze forming process is thus different from gravity, low pressureor high pressure casting in that solidification of the melt takes placeunder conditions of sustained temperature and pressure. Typically, thepressure applied to the melt may be of the order of 70 MPa and may besustained for a period of time of the order of 40 seconds. Thesolification parameters of the squeeze forming process are different tothose obtaining in other known casting processes and hence, inaccordance with the method of the invention, the desired microstructurecan be obtained at a solidification growth rate R of the solid phasefrom 1,000 to 2,500 μ/s whilst omitting zirconium and titanium from thecomposition. Specifically, titanium is not required as the desireddegree of grain refinement is found to be present in the as-formedarticle as a direct consequence of the squeeze forming method ofproduction.

Cylinder liners for an internal combustion engine were produced inaccordance with the invention and the microstructure of sections of oneliner are shown in the accompanying figures wherein:

FIG. 1 is a photomicrograph to a magnification of ×400 of an unetchedsection of the liner as formed, and

FIG. 2 is a photomicrograph to a magnification of ×400 of acorresponding unetched section of the liner after a full heat treatmentprocess (T6).

By way of example, and for test purposes, cylinder liners for anexternal combustion engine were produced by squeeze formingaluminium-silicon alloy material to the following two compositions byweight:

    ______________________________________                                         Example 1                                                                    ______________________________________                                        Silicon            14% to 16%                                                 Copper             1.9% to 2.2%                                               Nickel             1.0% to 1.4%                                               Magnesium          0.4% to 0.55%                                              Iron               0.6% to 1.0%                                               Manganese          0.3% to 0.6%                                               Strontium          0.02% to 0.08%                                             ______________________________________                                    

with the balance being aluminium and incidental impurities.

    ______________________________________                                         Example 2                                                                    ______________________________________                                        Silicon            14% to 16%                                                 Copper             1.9% to 2.2%                                               Nickel             1.0% to 1.4%                                               Magnesium          0.4% to 0.55%                                              Iron               0.6% to 1.0%                                               Manganese          0.3% to 0.6%                                               Strontium          0.02% to 0.1%                                              ______________________________________                                    

with the balance being aluminium and incidental impurities.

During the squeeze forming of cylinder liners to both of the abovecompositions the growth rate R of the solid phase during solidificationwas from 1,000 to 2,500 μ/s and the temperature gradient G at thesolid/liquid interface, expressed in °C./cm, was such that the ratio G/Rwas from 100 to 1,000 °Cs/cm².

In the as-formed cylinder liner, the microstructure shown in thephotomicrograph comprising FIG. 1 was found to be essentially eutecticcontaining not more than 10% of primary alpha-aluminium dendrites andbeing substantially free from intermetallic particles exceeding 10μ indiameter.

The as-formed liners were then given a full heat treatment process (T6)being a solution treatment of heating and holding the liners atapproximately 480° C. to 530° C. for between 5 and 20 hours, quenchingthe liners into hot water and then artificially ageing them by reheatingand holding at a temperature of around 140° C. to 250° C. for a timebetween two and 30 hours. The microstructure of such a heat treatedliner is shown in the photomicrograph comprising FIG. 2 and after thisfull heat treatment the average mechanical properties were found to be:

    ______________________________________                                        UTS                  350 to 380 MPa                                           Hardness (BHN)       130 to 160                                               0.2% Compressive YS  400 to 450 MPa.                                          ______________________________________                                    

Further elevated temperature mechanical testing was carried out whichyielded the following UTS values:

    ______________________________________                                        150° C.                                                                             1 hour       330 to 360 MPa                                                   1,000 hours  300 to 320 MPa                                      200° C.                                                                             1 hour       310 to 340 MPa                                                   1,000 hours  200 to 230 MPa                                      250° C.                                                                             1 hour       220 to 240 MPa                                                   1,000 hours  100 to 150 MPa                                      ______________________________________                                    

Under sliding wear conditions, the wear resistance properties ofcylinder liners produced as described above were found to be excellent.Thus the article of the present invention enjoys the inherent advantagesof a squeeze formed product in being produced to a high density withoutair entrapment in a relatively short production cycle time whilst alsobenefitting from the desirable properties of the essentially eutecticmicrostructure. Also, by utilisation of a squeeze forming process, theexpensive alloying elements zirconium and titanium can be omitted.

We claim:
 1. A squeeze formed aluminium-silicon alloy article producedfrom a melt consisting essentially of the following composition byweight:

    ______________________________________                                        Silicon             14% to 16%                                                Copper              1.9% to 2.2%                                              Nickel              1.0% to 1.4%                                              Magnesium           0.4% to 0.55%                                             Iron                0.6% to 1.0%                                              Manganese           0.3% to 0.6%                                              Silicon Modifier    0.02% to 0.1%                                             ______________________________________                                    

with the balance being aluminium and any unavoidable impurities, theas-formed article having an essentially eutectic microstructurecontaining not more than 10% of primary alpha-aluminium dendrites andbeing substantially free from intermetallic particles exceeding 10μ indiameter.
 2. A squeeze formed article as claimed in claim 1 wherein theweight of silicon modifier in the melt is within the range 0.02% to0.08%.
 3. A squeeze formed article as claimed in either one of claims 1or 2 wherein the silicon modifier is strontium.
 4. A squeeze formedarticle according to claim 1 or 2 comprising a cylinder liner for aninternal combustion engine.
 5. A squeeze formed article according toclaim 1 or 2 wherein the silicon modifier is strontium and the articlecomprises a cylinder liner for an internal combustion engine.
 6. Amethod comprising:producing a squeeze formed aluminium-silicon alloyarticle by forming a melt consisting essentially of the followingcomposition by weight:

    ______________________________________                                        Silicon             14% to 16%                                                Copper              1.9% to 2.2%                                              Nickel              1.0% to 1.4%                                              Magnesium           0.4% to 0.55%                                             Iron                0.6% to 1.0%                                              Manganese           0.3% to 0.6%                                              Silicon Modifier    0.02% to 0.1%                                             ______________________________________                                    

with the balance being aluminium and any unavoidable impurities;introducing the melt into a mould for the intended article; applyingsqueezing pressure to the melt in the mould and while maintaining themelt under squeezing pressure permitting the melt to solidify underconditions of sustained temperature and pressure such that the growthrate R of the solid phase during solidification is from 1,000 to 2,500μ/s and the temperature gradient G at the solid/liquid interface,expressed in °C./cm is such that the ratio G/R is from 100 to 1,000°Cs/cm² and the as-formed article has an essentially eutecticmicrostructure containing not more than 10% of primary alpha-aluminiumdendrites and is substantially free from intermetallic particlesexceeding 10) in diameter.
 7. A method according to claim 6comprising:subjecting the as-formed article to a solution heat treatmentprocess including heating and holding the article at a firsttemperature, water quenching the article and reheating and holding thearticle at a second temperature which is lower than said firsttemperature.
 8. A method according to claim 6 or 7 in which the articleis a cylinder liner for an internal combustion engine.
 9. A methodaccording to claim 6 or 7 in which the weight of silicon modifier in themelt is within the range of 0.02% to 0.08%.
 10. A method according toclaim 6 or 7 in which the silicon modifier is strontium.